STUDIES ON THE ROLE OF THE 2-OXONIA COPE REARRANGEMENT IN PI-CYCLIZATIONS OF ALPHA-METHOXYCARBONYL OXYCARBENIUM IONS

The 2-oxonia Cope rearrangement is shown to play an important role in Lewis acid-induced R-cyclization reactions of a variety of methyl 2-ac etoxy-2-(3-alken-I-oxy)acetates to 5- and 6-membered ring ethers. The influence of this [3,3]-sigmatropic rearrangement on the regio- and st ereochemical outcome of the cyclization is evaluated by studying three types of substrates, namely (1) four chain-substituted precursors, (2 ) three silicon-substituted precursors (allyl- and vinylsilanes) and ( 3) two enantiopure precursors. Controlling factors are the nature of t he ri-nucleophile and the substitution pattern of the oxycarbenium ton intermediate. The absolute stereochemistry is retained in the cycliza tion of the enantiopure substrates.